The solidification of molten metal is a dynamic process which is not fully understood, despite years of experimentation. In conventional casting technology, molten metal which has been heated to a temperature above its liquidus point is poured into a mold. Depending upon the particular casting process being utilized and the type of metal itself, varying amounts of superheat are required in order to insure that the metal will remain in a liquid state and therefore, flowable, during the casting or mold filling operation. The superheat, that amount of heat which causes the molten metal to exceed its liquidus temperature, begins to dissipate as soon as the molten metal is separated from the heat source. In recent years, industrial casting processes have tended to operate with as little superheat as possible, not only for energy conservation purposes, but for quality improvement. In all but the rarest cases, low superheat in the mold improves quality by reduced segregation, less shrinkage, lower gas content and other qualitative aspects. Unfortunately, the need for little or no superheat in the mold conflicts with the requirement for flowability during the mold filling phase. Therefore all commercial casting processes operate with some amount of superheat.
Because of the large amount of heat typically present in a molten metal, solidification normally takes place more slowly than would be desired. The mold absorbs heat from the molten metal and dissipates that heat to the surrounding atmosphere, often with an assist from some cooling apparatus. The outer surface of the molten metal adjacent to the mold loses its superheat most rapidly, and a solidifying metal shell quickly forms on the inner surface of the mold. However, the rate of heat loss between the liquidus and solidus temperatures of the metal is much slower than the rate of heat loss above liquidus or below solidus. Thus, the metal is at a temperature between liquidus and solidus for an extended period during solidification. It is during this time period that large dendrites begin to form at the liquid-solid interface.
Essentially all metals solidify by dendritic growth. Dendrites are "tree like" particles of solidifying metal which "grow" on the inner surface of the solidifying metal shell from the solid toward the liquid phase. In a static mold system, dendritic growth progresses from the coolest part of the system inward toward the hotter portion, and the solidification rate decreases exponentially as the shell thickens. This causes the growth of increasingly larger sized dendrites resulting in undesirably large sized grains in the solidified casting.
Castings often exhibit an undesireable characteristic called "piping" in which a continuous void is formed in the solidifying ingot or shape. Piping is caused by "bridging" of the dendrites across the final shrinking liquid core of the casting. The bridge is caused by a combination of thermal gradients and chemical segregation.
During solidification, the liquid phase of the metal is usually enriched in solute elements such as carbon, silicon, manganese, etc., by rejection of these elements from the solidifying phase into the liquid phase. This enriched liquid has a lower solidus temperature than the nominal composition of the melt, and will therefor solidify last.
Transverse temperature gradients can exist along the longitudinal centerline due to the shape of the casting or non-uniform cooling. These temperature differences can effect localized chemical composition and therefore alter the solidification temperature within a given region. Those regions will tend to "bridge", and "piping" will be formed between the bridges. These bridges block the flow of liquid metal resulting in centerline porosity or other voids in the solidified casting, which can seriously effect the strength and reliability of the final product.
Other problems in conventional casting can result from "inclusions" which become entrained in the melt. Inclusions are nonmetallic particles of a ceramic nature some of which are picked up from the furnace, ladle or any refractory surface with which the liquid comes into contact. Another source of nonmetallics is from elements in the liquid metal which combine with oxygen and nitrogen to form oxide and nitride compounds such as aluminium oxide (Al.sub.2 O.sub.3), various silica and manganese oxides. Other sources are from reactive elements which are added intentionally to reduce the oxygen and nitrogen content or otherwise impart specific properties to the metal. Sulphur, which is always present, even in trace amounts, may give rise to various sulphide compounds. These materials are extremely harmful to steel.
These inclusions have lower density than most liquid metal, and will float toward the upper surface. If the inclusions tend to stratify, (form a large grouping in one plane) a plane of weakness occurs which may result in failure of the finished part.
The importance of fluid flow in the control of solidification has been recognized for some time. Numerous devices are available for imparting motion to the liquid phase of solidifying metal along a solid/liquid interface during casting, including electromagnetically induced fluid flow and rotary stirring. These devices attempt to inhibit the growth of larger columnar dendrites, improve dispersion of chemical solutes, inhibit stratification of inclusions and reduce central porosity. It has been demonstrated that when the liquid phase of a solidifying casting is subjected to rapid motion, the grain structure will become modified. This modification can result in a reduction in grain size, shape and orientation as well as a decrease in chemical segregation. Non-metallics become more uniformly distributed and greater central soundness is achieved. Thermal gradients, magnetic coils, mechanical vibration and stirring have all been used in the prior art for the purpose of imparting motion to the liquid core. In general, prior art attempts to control solidification have required costly and cumbersome machinery to create fluid flow within the mold.
My prior U.S. Pat. No. Re 30,979, reissued on June 22, 1982, discloses a method and apparatus which partially solves the above problems by controlling the velocity of molten metal in a tubular mold to sweep the solid/liquid interface and thereby inhibit or reduce columnar dendritic growth. In that patent, an externally cooled inclined mold is supplied with molten metal at a controlled velocity. The velocity is such that the molten metal shears or breaks off the dendrites growing at the solid/liquid interface and entrains them in the molten metal eventually forming a slurry of dendritic particles and liquid metal. The final solidification of this slurry takes place rapidly avoiding piping and segregation, and producing an elongated bar having a finer grain structure than could be achieved by conventional techniques.
However, the invention disclosed and claimed in my reissue patent, discussed above, utilizes only elongated molds having a specified range of dimensions.
I have now applied the principles of my reissue U.S. Pat. No. 30,979, to achieve a further improvement in the quality of castings, and to provide more flexibility in the size, shape and type of castings which may be formed utilizing these principles.
It is a primary object of the present invention to thermally precondition molten metal to produce a molten metal slurry having a solids content for subsequent casting.
It is another object of the invention to control the rate of solidification of molten metal without the need for complex machinery.
Another object of the invention is to produce a slurry of molten metal generally having a temperature between the liquidus and solidus temperatures of the metal for subsequent casting into molds of varying shape.